It is known that substituted phenols undergo oxidative coupling to form self-condensation products such as diphenoquinone, 4,4'-dihydroxydiphenyl and polyphenylene oxide.
However, processes for producing 4,4'-dihydroxydiphenyls from alkylphenols as described in, for example, U.S. Pat. No. 4,180,686 and British Pat. No. 2,047,232 require an organic solvent or a stoichiometric amount of an organic reagent. In addition, in these processes, it is difficult to discontinue the oxidative coupling reaction at the stage of 4,4'-dihydroxydiphenyl, and the reaction proceeds to yield diphenoquinone or polyphenylene oxide as a major product.
On the other hand, Japanese Patent Application (OPI) No. 65834/78 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application") discloses a process for producing 4,4'-dihydroxydiphenyl using a copper-amine complex as a catalyst in water. However, this process requires a stirring apparatus having the performance of 6,000 to 10,000 rpm and a special apparatus of a wrinkled Morton flask. Thus, it is difficult to practice such a process on an industrial scale. When this special process is conducted using an industrially practicable apparatus as is employed in the present invention, the conversion of alkylphenol and the yield of 4,4'-dihydroxydiphenyl are low as shown in Comparative Example 1 described hereinafter and those are not satisfactory results.